In the related art, as an input circuit of an analog semiconductor integrated circuit, there is a rail-to-rail type input circuit that sets an input voltage level of a differential pair circuit to be within a wide range of voltages ranging from a power supply voltage to a ground voltage. A differential pair circuit including NMOS transistors and a differential pair circuit including PMOS transistors are combined, and a wide input voltage range of the input circuit is secured by using an output of the NMOS differential pair circuit when the input voltage level approximates the power supply voltage and using an output of the PMOS differential pair circuit when the input voltage level approximates the ground level.
When the rail-to-rail type input circuit is used as an input circuit of the comparator, one of two input signals may be a reference signal which is a DC bias signal, and the other may be a monitoring target signal. The comparator operates such that an output signal is inverted when the monitoring target signal is equal to or higher than, or is equal to or lower than, the reference signal.
There is a problem that the PMOS transistor that receives a DC reference signal having a low voltage level is likely to be influenced by negative-bias temperature instability (hereinafter, abbreviated to NBTI) degradation. In the PMOS differential pair circuit of the comparator, the NBTI degradation of the PMOS transistor which receives the DC reference signal is greater than that of the PMOS transistor that receives the monitoring target signal. Thus, the balance of the differential pair is lost by a difference between the degree of NBTI degradation of two inputs of the PMOS differential pair circuit, and thus, the detection accuracy of the comparator is deteriorated.